One of the current trends in the automobile industry is to lower vehicle weight to help achieve better fuel economy, thus helping to meet fuel economy standards and to offset the higher fuel prices. Lighter weight components are also attempted for easier manipulation by a user when desired. Another trend is that there is a broader range of vehicle models, which in turn reduces the volume of vehicles produced on a per model basis. Sport utility and crossover vehicles remain popular, and these typically include fairly heavy exterior body panels, such as rear liftgates, making these parts of the vehicle a target area for weight reduction. Such parts as liftgates for any vehicle are traditionally made from stamped steel panels that are heavy and have a high tooling cost. Traditional steel liftgates are expensive investments, heavy, take up a lot of OEM floor space in areas from the stamping plant, body shop, paint shop, and trim line. Further, steel liftgates have limited styling flexibility, take a lot of time to tool, and have corrosion concerns. Acrylonitrile Butadiene Styrene (ABS) is an alternative for larger parts because ABS is not as heat sensative or is not back-side read through prone. Sheet Molding Compound (SMC) is an alternative to steel for the inner and outer panels of the liftgate. Using SMC has several manufacturing concerns related to the material and process. Steel and SMC liftgates have a mass penalty over thermoplastics. There are also styling restrictions with traditional sheet metal components.
Thermoplastic polyolefin (TPO)—filled TPO (e.g., glass filled) or non-filled TPO—is another alternative material to steel and SMC but it is a difficult material to work with. TPO has been used for some parts but is thought of as being difficult to use because it is too soft, sensitive, flexible or difficult to process for making larger parts. The use of TPO has generally been limited to smaller parts. TPO has not been used for liftgate panels or other large exterior panels. TPO parts have also been generally limited to being welded first before painting Class A surfaces. Welding before painting is inefficient because it adds processing steps, e.g., powerwashing, masking, etc.
Thermoplastic composite type materials used for liftgate applications also have difficulty meeting customer performance specifications. More specifically some customer performance specifications include requirements that the liftgate prevent scattering of components at the time of the collision to the back door and the prevention of components or objects from passing through the liftgate ingressing or egressing from the vehicle interior. One of the problems that has been encountered is that pieces of the composite liftgate, when experiencing a crash are projected into the interior compartment of the vehicle. It is therefore desirable to prevent pieces, and particular large pieces of the liftgate from scattering into the interior compartment of the vehicle. It is also desirable to prevent objects from passing from the inside of the vehicle compartment through the liftgate. Traditionally liftgates have incorporated a steel cable or wire connected between the interior panel and exterior panel using fasteners. The steel cable serves as a tether to hold the components together even upon breaking. While this arrangement can work to pull together very large broken pieces of a liftgate at the time of collision, it does not prevent or stop larger pieces that shatter and form shards from being projected into the department at the time of the collision. In other words unless the liftgate breaks near an anchor point of the steel tether any shattering effect will not hold the shattered pieces together.
In another aspect of the invention it is desirable to develop new manufacturing techniques for creating a composite liftgate, or any other vehicle part, that provides sufficient strength, yet maintains or presents a desirable “Class-A” finish when viewed from the exterior. Multiple parts of any exterior body assembly must be joined together. Bonding is more typical for joining panels of these assemblies, but bonding is time consuming, expensive, very sensative, weaker and takes a lot of manufacturing space. By way of nonlimiting example, bonding systems require the following sensative perameters and steps: clean, flame, prime, adhesive, time, temperature, etc. Typical bonding uses adhesive, i.e., urethane adhesive, and/or a combination of adhesive and fasteners. Bonding, ultrasonic welding, or vibration welding of Class-A, painted thermoplastic automobile exterior parts are all particularly problematic and fail to provide required strength and surface appearance. Known attempts to try to overcome the problems with bonding have utilized ultrasonic welding, vibration welding (bulbs or wires), and are utilized on some spoiler applications, but there are issues with Class-A read through, sink marks, strength or structure weakness, limited shape allowance, the weld not being continuous, and not sealed, etc.
One of the current methods in automotive part manufacturing is ultrasonic welding or a combination of vibration and infrared welding of non-Class A parts. The combination creates debris. And vibration welding requires a smooth part curvature for moving back and forth while the weld is created.
Accordingly, for all these reasons, there exists a need for exterior body panels that are lightweight and meet strength requirements and surface appearance, and a thermoplastic welding method for exterior body panels that eliminates or reduces the problems or concerns associated with bonding systems.